In a conventional steering system such as for outboard motors used on boats, the propulsion unit having a stern drive, mounted on the transom of the boat, is pivoted about a vertical steering axis upon steering actuation by the operator at the helm. One typical steering system for a boat having a stern drive comprises a steering cable extending between the steering helm and the propulsion unit so that steering at the helm actuates the cable for causing steering movement of the propulsion unit about a steering axis. A conventional steering cable is the push-pull cable comprising a reciprocative inner core slidable in a protective, flexible outer sheath or housing. One end of the cable is actuably connected to the steering helm, and the other end is actuably connected to the steering mechanism of the propulsion unit. When the wheel is turned at the helm, the cable is actuated by a push-pull movement of the inner core, thereby causing a steering movement of the propulsion unit. Hydraulic activated steering means can be used in place of the cable steering, wherein hydraulic fluid, e.g. oil, is pumped from the steering helm through conduits to a cylinder-piston control means in response to rotation of the steering wheel in one direction or the other. Actuation of the control means actuates the steering mechanism of the propulsion unit, thereby turning the propulsion unit in a common direction.
Prior art teaching steering systems of this type include the following U.S. Pat. Nos. 4,592,732; 4,615,290; 4,632,049; 4,568,292; and 4,295,833. Additionally, British Patent Application 2,159,483A discloses a power steering system for an outboard having a hydraulic cylinder-piston assembly and a control valve which is operated by an actuator including a push-pull cable to selectively extend and retract the piston rod and effect steering of the propulsion unit. The power steering assist system as taught in each of the prior art patents and British application identified above, however, is mounted onto and supported by the propulsion unit. Mounting the power steering system on the propulsion unit is disadvantageous for a number of reasons. First, the propulsion unit mounting position must be changed because there is a steering apparatus to conflict with the boat transom design during verticle tilt movement. In order to mount the power steering system to the propulsion unit, special bracketry is required for each engine design, because the mounting pads vary markedly depending on the design. Exemplary of engine mounting is the disclosure in the above identified British Patent Application, where, as shown in FIG. 7, the power assist unit 120 is mounted on a propulsion unit 10, which is mounted to a boat transom 22. As the propulsion unit 10 tilts about the horizontal axis 42, the power assist unit 120 may come into contact with the boat transom, thereby limiting its applicability. A second disadvantage is that the power steering system, including the supply and return lines which are under high pressure, are subjected to sun rays, salt water corrosion, and physical abuse because of exposure. Thirdly, such designs as shown in the prior art do not allow for steering shock to be absorbed partially by the steering cable, in that any steering shock is prevented from passing beyond the power assist steering system causing a high stress on the propulsion unit steering components. Lastly, the systems of the prior art, and in particular such a system as taught by the aforesaid British Patent Application, are designed to continuously supply fluid to the system, and not just when steering movement occurs. This constant fluid supply system wastes propulsion engine horsepower.
U.S. Pat. No. 497,706 discloses an in-line steering assist system in that the system is mounted remote from the rudder disposed adjacent the propeller. A retractable carriage is moved by a fluid actuated piston, and a cable extending from the piston, around pulleys on the carriage and to the rudder disk, moves the disk in response to movement of the piston. Little or no torque created at the rudder is consumed by the steering assist means to thereby reduce the steering effort required at the helm.